how do you interrupt it ?
i find if the image is a large image it will chug for 20 seconds sometimes and the esc key will not work .. i can not change values while the bar is in progress, a small icon near the end of the progress bar to cancel would not be all that difficult to put in there ..

Just do something -- change a number, hit a button, etc. There should be no need for a cancel button in most operations - which is why I keep asking you to tell us what you are going that is making SaveForWeb non-responsive.